Method and apparatus for performing device-to-device discovery in wireless communication system

ABSTRACT

A method and apparatus for performing device-to-device (D2D) discovery in a wireless communication system is provided. A first user equipment (UE), which supports proximity services (ProSe), transmits a request of D2D discovery to a network, and receives information on a second UE, which supports the ProSe and is located in proximity of the first UE, and information on D2D discovery signaling. The first UE performs D2D discovery based on the received information on the second UE and information on D2D discovery signaling.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the National Phase of PCT/KR2014/007335 filed on Aug. 7, 2014, which claims priority under 35 U.S.C. 119(e) to U.S. Provisional Application No. 61/863,386 filed on Aug. 7, 2013, all of which are hereby expressly incorporated by reference into the present application.

TECHNICAL FIELD

The present invention relates to wireless communications, and more specifically, to a method and apparatus for performing a device-to-device (D2D) discovery in a wireless communication system.

BACKGROUND ART

Universal mobile telecommunications system (UMTS) is a 3^(rd) generation (3G) asynchronous mobile communication system operating in wideband code division multiple access (WCDMA) based on European systems, global system for mobile communications (GSM) and general packet radio services (GPRS). A long-term evolution (LTE) of UMTS is under discussion by the 3rd generation partnership project (3GPP) that standardized UMTS.

The 3GPP LTE is a technology for enabling high-speed packet communications. Many schemes have been proposed for the LTE objective including those that aim to reduce user and provider costs, improve service quality, and expand and improve coverage and system capacity. The 3GPP LTE requires reduced cost per bit, increased service availability, flexible use of a frequency band, a simple structure, an open interface, and adequate power consumption of a terminal as an upper-level requirement.

Recently, there has been a surge of interest in supporting direct device-to-device (D2D) communication. This new interest is motivated by several factors, including the popularity of proximity-based services, driven largely by social networking applications, and the crushing data demands on cellular spectrum, much of which is localized traffic, and the under-utilization of uplink frequency bands. 3GPP is targeting the availability of D2D communication in LTE rel-12 to enable LTE become a competitive broadband communication technology for public safety networks, used by first responders. Due to the legacy issues and budget constraints, current public safety networks are still mainly based on obsolete 2G technologies while commercial networks are rapidly migrating to LTE. This evolution gap and the desire for enhanced services have led to global attempts to upgrade existing public safety networks. Compared to commercial networks, public safety networks have much more stringent service requirements (e.g., reliability and security) and also require direct communication, especially when cellular coverage fails or is not available. This essential direct mode feature is currently missing in LTE.

From a technical perspective, exploiting the nature proximity of communicating devices may provide multiple performance benefits. First, D2D user equipments (UEs) may enjoy high data rate and low end-to-end delay due to the short-range direct communication. Second, it is more resource-efficient for proximate UEs to communicate directly with each other, versus routing through an evolved NodeB (eNB) and possibly the core network. In particular, compared to normal downlink/uplink cellular communication, direct communication saves energy and improves radio resource utilization. Third, switching from an infrastructure path to a direct path offloads cellular traffic, alleviating congestion, and thus benefitting other non-D2D UEs as well. Other benefits may be envisioned such as range extension via UE-to-UE relaying.

From an economic perspective, LTE D2D should create new business opportunities, though its commercial applications are not the focus in LTE rel-12. For example, many social networking applications rely on the ability to discover users that are in proximity, but the device discovery processes typically work in a non-autonomous manner. Users first register their location information in a central server once launching the application. The central server then distributes the registered location information to other users using the application. It would be appealing to the service providers if device discovery can work autonomously without manual location registration. Other examples include e-commerce, whereby private information need only be shared locally between two parties, and large file transfers, e.g., just-taken video clips shared amongst other nearby friends.

Thus far, use cases of 3GPP proximity services (ProSe) and corresponding architecture enhancements have been specified and studied. ProSe direct discovery is defined as the procedure used by the ProSe-enabled UE to discover other ProSe-enabled UE(s) in its proximity using evolved universal terrestrial radio access (E-UTRA) direct radio signals via PC5 interface. A method for performing ProSe discovery effectively may be required.

SUMMARY OF INVENTION Technical Problem

The present invention provides a method and apparatus for performing a device-to-device (D2D) discovery in a wireless communication system. The present invention provides a method for receiving information on a proximity services (ProSe)-enabled user equipment (UE) and D2D discovery signaling, and performing D2D discovery with the ProSe-enabled UE.

Solution to Problem

In an aspect, a method for performing, by a first user equipment (UE) which supports proximity services (ProSe), device-to-device (D2D) discovery in a wireless communication system is provided. The method includes transmitting a request of D2D discovery to a network, receiving information on a second UE, which supports the ProSe and is located in proximity of the first UE, and information on D2D discovery signaling, and performing D2D discovery based on the received information on the second UE and information on D2D discovery signaling.

In another aspect, a first user equipment (UE) which supports proximity services (ProSe) in a wireless communication system is provided. The first UE includes a radio frequency (RF) unit for transmitting or receiving a radio signal, and a processor coupled to the RF unit, and configured to transmit a request of D2D discovery to a network, receive information on a second UE, which supports the ProSe and is located in proximity of the first UE, and information on D2D discovery signaling, and perform D2D discovery based on the received information on the second UE and information on D2D discovery signaling.

Advantageous Effects of Invention

In radio resource control (RRC) connected mode, D2D discovery can be performed effectively.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows LTE system architecture.

FIG. 2 shows a block diagram of architecture of a typical E-UTRAN and a typical EPC.

FIG. 3 shows a block diagram of a user plane protocol stack and a control plane protocol stack of an LTE system.

FIG. 4 shows an example of a physical channel structure.

FIG. 5 and FIG. 6 show ProSe direct communication scenarios without a relay.

FIG. 7 shows reference architecture for ProSe.

FIG. 8 shows ProSe architecture for network based solution for ProSe discovery.

FIG. 9 shows a signaling flow for Prose registration procedure.

FIG. 10 shows a signaling flow for targeted ProSe discovery procedure.

FIG. 11 shows an example of a method for performing D2D discovery according to an embodiment of the present invention.

FIG. 12 shows another example of a method for performing D2D discovery according to an embodiment of the present invention.

FIG. 13 is a block diagram showing wireless communication system to implement an embodiment of the present invention.

MODE FOR THE INVENTION

The technology described below can be used in various wireless communication systems such as code division multiple access (CDMA), frequency division multiple access (FDMA), time division multiple access (TDMA), orthogonal frequency division multiple access (OFDMA), single carrier frequency division multiple access (SC-FDMA), etc. The CDMA can be implemented with a radio technology such as universal terrestrial radio access (UTRA) or CDMA-2000. The TDMA can be implemented with a radio technology such as global system for mobile communications (GSM)/general packet ratio service (GPRS)/enhanced data rate for GSM evolution (EDGE). The OFDMA can be implemented with a radio technology such as institute of electrical and electronics engineers (IEEE) 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20, evolved UTRA (E-UTRA), etc. IEEE 802.16m is evolved from IEEE 802.16e, and provides backward compatibility with a system based on the IEEE 802.16e. The UTRA is a part of a universal mobile telecommunication system (UMTS). 3^(rd) generation partnership project (3GPP) long term evolution (LTE) is a part of an evolved UMTS (E-UMTS) using the E-UTRA. The 3GPP LTE uses the OFDMA in a downlink and uses the SC-FDMA in an uplink LTE-advanced (LTE-A) is an evolution of the LTE.

For clarity, the following description will focus on LTE-A. However, technical features of the present invention are not limited thereto.

FIG. 1 shows LTE system architecture. The communication network is widely deployed to provide a variety of communication services such as voice over internet protocol (VoIP) through IMS and packet data.

Referring to FIG. 1, the LTE system architecture includes one or more user equipment (UE; 10), an evolved-UMTS terrestrial radio access network (E-UTRAN) and an evolved packet core (EPC). The UE 10 refers to a communication equipment carried by a user. The UE 10 may be fixed or mobile, and may be referred to as another terminology, such as a mobile station (MS), a user terminal (UT), a subscriber station (SS), a wireless device, etc.

The E-UTRAN includes one or more evolved node-B (eNB) 20, and a plurality of UEs may be located in one cell. The eNB 20 provides an end point of a control plane and a user plane to the UE 10. The eNB 20 is generally a fixed station that communicates with the UE 10 and may be referred to as another terminology, such as a base station (BS), a base transceiver system (BTS), an access point, etc. One eNB 20 may be deployed per cell. There are one or more cells within the coverage of the eNB 20. A single cell is configured to have one of bandwidths selected from 1.25, 2.5, 5, 10, and 20 MHz, etc., and provides downlink or uplink transmission services to several UEs. In this case, different cells can be configured to provide different bandwidths.

Hereinafter, a downlink (DL) denotes communication from the eNB 20 to the UE 10, and an uplink (UL) denotes communication from the UE 10 to the eNB 20. In the DL, a transmitter may be a part of the eNB 20, and a receiver may be a part of the UE 10. In the UL, the transmitter may be a part of the UE 10, and the receiver may be a part of the eNB 20.

The EPC includes a mobility management entity (MME) which is in charge of control plane functions, and a system architecture evolution (SAE) gateway (S-GW) which is in charge of user plane functions. The MME/S-GW 30 may be positioned at the end of the network and connected to an external network. The MME has UE access information or UE capability information, and such information may be primarily used in UE mobility management. The S-GW is a gateway of which an endpoint is an E-UTRAN. The MME/S-GW 30 provides an end point of a session and mobility management function for the UE 10. The EPC may further include a packet data network (PDN) gateway (PDN-GW). The PDN-GW is a gateway of which an endpoint is a PDN.

The MME provides various functions including non-access stratum (NAS) signaling to eNBs 20, NAS signaling security, access stratum (AS) security control, Inter core network (CN) node signaling for mobility between 3GPP access networks, idle mode UE reachability (including control and execution of paging retransmission), tracking area list management (for UE in idle and active mode), P-GW and S-GW selection, MME selection for handovers with MME change, serving GPRS support node (SGSN) selection for handovers to 2G or 3G 3GPP access networks, roaming, authentication, bearer management functions including dedicated bearer establishment, support for public warning system (PWS) (which includes earthquake and tsunami warning system (ETWS) and commercial mobile alert system (CMAS)) message transmission. The S-GW host provides assorted functions including per-user based packet filtering (by e.g., deep packet inspection), lawful interception, UE Internet protocol (IP) address allocation, transport level packet marking in the DL, UL and DL service level charging, gating and rate enforcement, DL rate enforcement based on APN-AMBR. For clarity MME/S-GW 30 will be referred to herein simply as a “gateway,” but it is understood that this entity includes both the MME and S-GW.

Interfaces for transmitting user traffic or control traffic may be used. The UE 10 and the eNB 20 are connected by means of a Uu interface. The eNBs 20 are interconnected by means of an X2 interface. Neighboring eNBs may have a meshed network structure that has the X2 interface. The eNBs 20 are connected to the EPC by means of an S1 interface. The eNBs 20 are connected to the MME by means of an S1-MME interface, and are connected to the S-GW by means of S1-U interface. The S1 interface supports a many-to-many relation between the eNB 20 and the MME/S-GW.

FIG. 2 shows a block diagram of architecture of a typical E-UTRAN and a typical EPC. Referring to FIG. 2, the eNB 20 may perform functions of selection for gateway 30, routing toward the gateway 30 during a radio resource control (RRC) activation, scheduling and transmitting of paging messages, scheduling and transmitting of broadcast channel (BCH) information, dynamic allocation of resources to the UEs 10 in both UL and DL, configuration and provisioning of eNB measurements, radio bearer control, radio admission control (RAC), and connection mobility control in LTE_ACTIVE state. In the EPC, and as noted above, gateway 30 may perform functions of paging origination, LTE_IDLE state management, ciphering of the user plane, SAE bearer control, and ciphering and integrity protection of NAS signaling.

FIG. 3 shows a block diagram of a user plane protocol stack and a control plane protocol stack of an LTE system. FIG. 3-(a) shows a block diagram of a user plane protocol stack of an LTE system, and FIG. 3-(b) shows a block diagram of a control plane protocol stack of an LTE system.

Layers of a radio interface protocol between the UE and the E-UTRAN may be classified into a first layer (L1), a second layer (L2), and a third layer (L3) based on the lower three layers of the open system interconnection (OSI) model that is well-known in the communication system. The radio interface protocol between the UE and the E-UTRAN may be horizontally divided into a physical layer, a data link layer, and a network layer, and may be vertically divided into a control plane (C-plane) which is a protocol stack for control signal transmission and a user plane (U-plane) which is a protocol stack for data information transmission. The layers of the radio interface protocol exist in pairs at the UE and the E-UTRAN, and are in charge of data transmission of the Uu interface.

A physical (PHY) layer belongs to the L1. The PHY layer provides a higher layer with an information transfer service through a physical channel. The PHY layer is connected to a medium access control (MAC) layer, which is a higher layer of the PHY layer, through a transport channel. A physical channel is mapped to the transport channel. Data is transferred between the MAC layer and the PHY layer through the transport channel. Between different PHY layers, i.e., a PHY layer of a transmitter and a PHY layer of a receiver, data is transferred through the physical channel using radio resources. The physical channel is modulated using an orthogonal frequency division multiplexing (OFDM) scheme, and utilizes time and frequency as a radio resource.

The PHY layer uses several physical control channels. A physical downlink control channel (PDCCH) reports to a UE about resource allocation of a paging channel (PCH) and a downlink shared channel (DL-SCH), and hybrid automatic repeat request (HARM) information related to the DL-SCH. The PDCCH may carry a UL grant for reporting to the UE about resource allocation of UL transmission. A physical control format indicator channel (PCFICH) reports the number of OFDM symbols used for PDCCHs to the UE, and is transmitted in every subframe. A physical hybrid ARQ indicator channel (PHICH) carries an HARQ acknowledgement (ACK)/non-acknowledgement (NACK) signal in response to UL transmission. A physical uplink control channel (PUCCH) carries UL control information such as HARQ ACK/NACK for DL transmission, scheduling request, and CQI. A physical uplink shared channel (PUSCH) carries a UL-uplink shared channel (SCH).

FIG. 4 shows an example of a physical channel structure. A physical channel consists of a plurality of subframes in time domain and a plurality of subcarriers in frequency domain. One subframe consists of a plurality of symbols in the time domain. One subframe consists of a plurality of resource blocks (RBs). One RB consists of a plurality of symbols and a plurality of subcarriers. In addition, each subframe may use specific subcarriers of specific symbols of a corresponding subframe for a PDCCH. For example, a first symbol of the subframe may be used for the PDCCH. The PDCCH carries dynamic allocated resources, such as a physical resource block (PRB) and modulation and coding scheme (MCS). A transmission time interval (TTI) which is a unit time for data transmission may be equal to a length of one subframe. The length of one subframe may be 1 ms.

The transport channel is classified into a common transport channel and a dedicated transport channel according to whether the channel is shared or not. A DL transport channel for transmitting data from the network to the UE includes a broadcast channel (BCH) for transmitting system information, a paging channel (PCH) for transmitting a paging message, a DL-SCH for transmitting user traffic or control signals, etc. The DL-SCH supports HARQ, dynamic link adaptation by varying the modulation, coding and transmit power, and both dynamic and semi-static resource allocation. The DL-SCH also may enable broadcast in the entire cell and the use of beamforming. The system information carries one or more system information blocks. All system information blocks may be transmitted with the same periodicity. Traffic or control signals of a multimedia broadcast/multicast service (MBMS) may be transmitted through the DL-SCH or a multicast channel (MCH).

A UL transport channel for transmitting data from the UE to the network includes a random access channel (RACH) for transmitting an initial control message, a UL-SCH for transmitting user traffic or control signals, etc. The UL-SCH supports HARQ and dynamic link adaptation by varying the transmit power and potentially modulation and coding. The UL-SCH also may enable the use of beamforming. The RACH is normally used for initial access to a cell.

A MAC layer belongs to the L2. The MAC layer provides services to a radio link control (RLC) layer, which is a higher layer of the MAC layer, via a logical channel. The MAC layer provides a function of mapping multiple logical channels to multiple transport channels. The MAC layer also provides a function of logical channel multiplexing by mapping multiple logical channels to a single transport channel. A MAC sublayer provides data transfer services on logical channels.

The logical channels are classified into control channels for transferring control plane information and traffic channels for transferring user plane information, according to a type of transmitted information. That is, a set of logical channel types is defined for different data transfer services offered by the MAC layer. The logical channels are located above the transport channel, and are mapped to the transport channels.

The control channels are used for transfer of control plane information only. The control channels provided by the MAC layer include a broadcast control channel (BCCH), a paging control channel (PCCH), a common control channel (CCCH), a multicast control channel (MCCH) and a dedicated control channel (DCCH). The BCCH is a downlink channel for broadcasting system control information. The PCCH is a downlink channel that transfers paging information and is used when the network does not know the location cell of a UE. The CCCH is used by UEs having no RRC connection with the network. The MCCH is a point-to-multipoint downlink channel used for transmitting MBMS control information from the network to a UE. The DCCH is a point-to-point bi-directional channel used by UEs having an RRC connection that transmits dedicated control information between a UE and the network.

Traffic channels are used for the transfer of user plane information only. The traffic channels provided by the MAC layer include a dedicated traffic channel (DTCH) and a multicast traffic channel (MTCH). The DTCH is a point-to-point channel, dedicated to one UE for the transfer of user information and can exist in both uplink and downlink. The MTCH is a point-to-multipoint downlink channel for transmitting traffic data from the network to the UE.

Uplink connections between logical channels and transport channels include the DCCH that can be mapped to the UL-SCH, the DTCH that can be mapped to the UL-SCH and the CCCH that can be mapped to the UL-SCH. Downlink connections between logical channels and transport channels include the BCCH that can be mapped to the BCH or DL-SCH, the PCCH that can be mapped to the PCH, the DCCH that can be mapped to the DL-SCH, and the DTCH that can be mapped to the DL-SCH, the MCCH that can be mapped to the MCH, and the MTCH that can be mapped to the MCH.

An RLC layer belongs to the L2. The RLC layer provides a function of adjusting a size of data, so as to be suitable for a lower layer to transmit the data, by concatenating and segmenting the data received from a higher layer in a radio section. In addition, to ensure a variety of quality of service (QoS) required by a radio bearer (RB), the RLC layer provides three operation modes, i.e., a transparent mode (TM), an unacknowledged mode (UM), and an acknowledged mode (AM). The AM RLC provides a retransmission function through an automatic repeat request (ARQ) for reliable data transmission. Meanwhile, a function of the RLC layer may be implemented with a functional block inside the MAC layer. In this case, the RLC layer may not exist.

A packet data convergence protocol (PDCP) layer belongs to the L2. The PDCP layer provides a function of header compression function that reduces unnecessary control information such that data being transmitted by employing IP packets, such as IPv4 or IPv6, can be efficiently transmitted over a radio interface that has a relatively small bandwidth. The header compression increases transmission efficiency in the radio section by transmitting only necessary information in a header of the data. In addition, the PDCP layer provides a function of security. The function of security includes ciphering which prevents inspection of third parties, and integrity protection which prevents data manipulation of third parties.

A radio resource control (RRC) layer belongs to the L3. The RLC layer is located at the lowest portion of the L3, and is only defined in the control plane. The RRC layer takes a role of controlling a radio resource between the UE and the network. For this, the UE and the network exchange an RRC message through the RRC layer. The RRC layer controls logical channels, transport channels, and physical channels in relation to the configuration, reconfiguration, and release of RBs. An RB is a logical path provided by the L1 and L2 for data delivery between the UE and the network. That is, the RB signifies a service provided the L2 for data transmission between the UE and E-UTRAN. The configuration of the RB implies a process for specifying a radio protocol layer and channel properties to provide a particular service and for determining respective detailed parameters and operations. The RB is classified into two types, i.e., a signaling RB (SRB) and a data RB (DRB). The SRB is used as a path for transmitting an RRC message in the control plane. The DRB is used as a path for transmitting user data in the user plane.

Referring to FIG. 3-(a), the RLC and MAC layers (terminated in the eNB on the network side) may perform functions such as scheduling, automatic repeat request (ARQ), and hybrid automatic repeat request (HARM). The PDCP layer (terminated in the eNB on the network side) may perform the user plane functions such as header compression, integrity protection, and ciphering.

Referring to FIG. 3-(b), the RLC and MAC layers (terminated in the eNB on the network side) may perform the same functions for the control plane. The RRC layer (terminated in the eNB on the network side) may perform functions such as broadcasting, paging, RRC connection management, RB control, mobility functions, and UE measurement reporting and controlling. The NAS control protocol (terminated in the MME of gateway on the network side) may perform functions such as a SAE bearer management, authentication, LTE_IDLE mobility handling, paging origination in LTE_IDLE, and security control for the signaling between the gateway and UE.

An RRC state indicates whether an RRC layer of the UE is logically connected to an RRC layer of the E-UTRAN. The RRC state may be divided into two different states such as an RRC_connected state and an RRC idle state. When an RRC connection is established between the RRC layer of the UE and the RRC layer of the E-UTRAN, the UE is in RRC_CONNECTED, and otherwise the UE is in RRC_IDLE. Since the UE in RRC_CONNECTED has the RRC connection established with the E-UTRAN, the E-UTRAN may recognize the existence of the UE in RRC_CONNECTED and may effectively control the UE. Meanwhile, the UE in RRC_IDLE may not be recognized by the E-UTRAN, and a CN manages the UE in unit of a TA which is a larger area than a cell. That is, only the existence of the UE in RRC_IDLE is recognized in unit of a large area, and the UE must transition to RRC_CONNECTED to receive a typical mobile communication service such as voice or data communication.

In RRC_IDLE state, the UE may receive broadcasts of system information and paging information while the UE specifies a discontinuous reception (DRX) configured by NAS, and the UE has been allocated an identification (ID) which uniquely identifies the UE in a tracking area and may perform public land mobile network (PLMN) selection and cell re-selection. Also, in RRC_IDLE state, no RRC context is stored in the eNB.

In RRC_CONNECTED state, the UE has an E-UTRAN RRC connection and a context in the E-UTRAN, such that transmitting and/or receiving data to/from the eNB becomes possible. Also, the UE can report channel quality information and feedback information to the eNB. In RRC_CONNECTED state, the E-UTRAN knows the cell to which the UE belongs. Therefore, the network can transmit and/or receive data to/from UE, the network can control mobility (handover and inter-radio access technologies (RAT) cell change order to GSM EDGE radio access network (GERAN) with network assisted cell change (NACC)) of the UE, and the network can perform cell measurements for a neighboring cell.

In RRC_IDLE state, the UE specifies the paging DRX cycle. Specifically, the UE monitors a paging signal at a specific paging occasion of every UE specific paging DRX cycle. The paging occasion is a time interval during which a paging signal is transmitted. The UE has its own paging occasion.

A paging message is transmitted over all cells belonging to the same tracking area. If the UE moves from one TA to another TA, the UE will send a tracking area update (TAU) message to the network to update its location.

When the user initially powers on the UE, the UE first searches for a proper cell and then remains in RRC_IDLE in the cell. When there is a need to establish an RRC connection, the UE which remains in RRC_IDLE establishes the RRC connection with the RRC of the E-UTRAN through an RRC connection procedure and then may transition to RRC_CONNECTED. The UE which remains in RRC_IDLE may need to establish the RRC connection with the E-UTRAN when uplink data transmission is necessary due to a user's call attempt or the like or when there is a need to transmit a response message upon receiving a paging message from the E-UTRAN.

It is known that different cause values may be mapped o the signature sequence used to transmit messages between a UE and eNB and that either channel quality indicator (CQI) or path loss and cause or message size are candidates for inclusion in the initial preamble.

When a UE wishes to access the network and determines a message to be transmitted, the message may be linked to a purpose and a cause value may be determined. The size of the ideal message may be also be determined by identifying all optional information and different alternative sizes, such as by removing optional information, or an alternative scheduling request message may be used.

The UE acquires necessary information for the transmission of the preamble, UL interference, pilot transmit power and required signal-to-noise ratio (SNR) for the preamble detection at the receiver or combinations thereof. This information must allow the calculation of the initial transmit power of the preamble. It is beneficial to transmit the UL message in the vicinity of the preamble from a frequency point of view in order to ensure that the same channel is used for the transmission of the message.

The UE should take into account the UL interference and the UL path loss in order to ensure that the network receives the preamble with a minimum SNR. The UL interference can be determined only in the eNB, and therefore, must be broadcast by the eNB and received by the UE prior to the transmission of the preamble. The UL path loss can be considered to be similar to the DL path loss and can be estimated by the UE from the received RX signal strength when the transmit power of some pilot sequence of the cell is known to the UE.

The required UL SNR for the detection of the preamble would typically depend on the eNB configuration, such as a number of Rx antennas and receiver performance. There may be advantages to transmit the rather static transmit power of the pilot and the necessary UL SNR separately from the varying UL interference and possibly the power offset required between the preamble and the message.

The initial transmission power of the preamble can be roughly calculated according to the following formula: Transmit power=TransmitPilot−RxPilot+ULInterference+Offset+SNRRequired

Therefore, any combination of SNRRequired, ULInterference, TransmitPilot and Offset can be broadcast. In principle, only one value must be broadcast. This is essentially in current UMTS systems, although the UL interference in 3GPP LTE will mainly be neighboring cell interference that is probably more constant than in UMTS system.

The UE determines the initial UL transit power for the transmission of the preamble as explained above. The receiver in the eNB is able to estimate the absolute received power as well as the relative received power compared to the interference in the cell. The eNB will consider a preamble detected if the received signal power compared to the interference is above an eNB known threshold.

The UE performs power ramping in order to ensure that a UE can be detected even if the initially estimated transmission power of the preamble is not adequate. Another preamble will most likely be transmitted if no ACK or NACK is received by the UE before the next random access attempt. The transmit power of the preamble can be increased, and/or the preamble can be transmitted on a different UL frequency in order to increase the probability of detection. Therefore, the actual transmit power of the preamble that will be detected does not necessarily correspond to the initial transmit power of the preamble as initially calculated by the UE.

The UE must determine the possible UL transport format. The transport format, which may include MCS and a number of resource blocks that should be used by the UE, depends mainly on two parameters, specifically the SNR at the eNB and the required size of the message to be transmitted.

In practice, a maximum UE message size, or payload, and a required minimum SNR correspond to each transport format. In UMTS, the UE determines before the transmission of the preamble whether a transport format can be chosen for the transmission according to the estimated initial preamble transmit power, the required offset between preamble and the transport block, the maximum allowed or available UE transmit power, a fixed offset and additional margin. The preamble in UMTS need not contain any information regarding the transport format selected by the EU since the network does not need to reserve time and frequency resources and, therefore, the transport format is indicated together with the transmitted message.

The eNB must be aware of the size of the message that the UE intends to transmit and the SNR achievable by the UE in order to select the correct transport format upon reception of the preamble and then reserve the necessary time and frequency resources. Therefore, the eNB cannot estimate the SNR achievable by the EU according to the received preamble because the UE transmit power compared to the maximum allowed or possible UE transmit power is not known to the eNB, given that the UE will most likely consider the measured path loss in the DL or some equivalent measure for the determination of the initial preamble transmission power.

The eNB could calculate a difference between the path loss estimated in the DL compared and the path loss of the UL. However, this calculation is not possible if power ramping is used and the UE transmit power for the preamble does not correspond to the initially calculated UE transmit power. Furthermore, the precision of the actual UE transmit power and the transmit power at which the UE is intended to transmit is very low. Therefore, it has been proposed to code the path loss or CQI estimation of the downlink and the message size or the cause value in the UL in the signature.

Proximity Services (ProSe) are described. It may be refer to 3GPP TR 23.703 V0.4.1 (2013-06). The ProSe may be a concept including a device-to-device (D2D) communication. Hereinafter, the ProSe may be used by being mixed with a device-to-device (D2D).

ProSe direct communication means a communication between two or more UEs in proximity that are ProSe-enabled, by means of user plane transmission using E-UTRA technology via a path not traversing any network node. ProSe-enabled UE means a UE that supports ProSe requirements and associated procedures. Unless explicitly stated otherwise, a ProSe-enabled UE refers both to a non-public safety UE and a public safety UE. ProSe-enabled public safety UE means a ProSe-enabled UE that also supports ProSe procedures and capabilities specific to public safety. ProSe-enabled non-public safety UE means a UE that supports ProSe procedures and but not capabilities specific to public safety. ProSe direct discovery means a procedure employed by a ProSe-enabled UE to discover other ProSe-enabled UEs in its vicinity by using only the capabilities of the two UEs with 3GPP LTE rel-12 E-UTRA technology. EPC-level ProSe discovery means a process by which the EPC determines the proximity of two ProSe-enabled UEs and informs them of their proximity.

When the registered public land mobile network (PLMN), ProSe direct communication path and coverage status (in coverage or out of coverage) are considered, there are a number of different possible scenarios. Different combinations of direct data paths and in-coverage and out-of-coverage may be considered.

FIG. 5 and FIG. 6 show ProSe direct communication scenarios without a relay. FIG. 5-(a) shows a case that UE1 and UE2 are out of coverage. FIG. 5-(b) shows a case that UE1 is in coverage and in PLMN A, and UE2 is out of coverage. FIG. 5-(c) shows a case that UE1 and UE2 are in coverage and in PLMN A, and UE1 and UE2 shares the same PLMN A and the same cell. FIG. 5-(d) shows a case that UE1 and UE2 are in coverage and in the same PLMN A, but UE1 and UE2 are in different cells each other. FIG. 6-(a) shows a case that UE1 and UE2 are in coverage, but UE1 and UE2 are in different PLMNs (i.e., PLMN A/B) and different cells each other. UE1 and UE2 are in both cells' coverage. FIG. 6-(b) shows a case that UE1 and UE2 are in coverage, but UE1 and UE2 are in different PLMNs (i.e., PLMN A/B) and different cells each other. UE1 is in both cells' coverage and UE2 is in serving cell's coverage. FIG. 6-(c) shows a case that UE1 and UE2 are in coverage, but UE1 and UE2 are in different PLMNs (i.e., PLMN A/B) and different cells each other. UE1 and UE2 are in its own serving cell's coverage. In the description above, “in coverage and in PLMN A” means that the UE is camping on the cell of the PLMN A and under the control of the PLMN A.

The ProSe direct communication scenarios described above may not cover all possible ProSe communication scenarios. Additional scenarios could be added. The ProSe direct communication scenarios described above are all applicable in cases of network sharing. It is for further study whether the scenarios described above could be enhanced or additional figures should to be added to further clarify the scenarios in relation to network sharing. It is for further study whether ProSe direct communication scenarios described above should be enhanced or new ProSe communication scenarios should be added for group communication. It is for further study whether ProSe direct communication scenarios described above should be enhanced or new ProSe communication scenarios should be added to cover further roaming cases.

Two different modes for ProSe direct communication one-to-one may be supported.

-   -   Network independent direct communication: This mode of operation         for ProSe direct communication does not require any network         assistance to authorize the connection and communication is         performed by using only functionality and information local to         the UE. This mode is applicable only to pre-authorized         ProSe-enabled public safety UEs, regardless of whether the UEs         are served by E-UTRAN or not.     -   Network authorized direct communication: This mode of operation         for ProSe direct communication always requires network         assistance and may also be applicable when only one UE is         “served by E-UTRAN” for public safety UEs. For non-public safety         UEs both UEs must be “served by E-UTRAN”.

FIG. 7 shows reference architecture for ProSe. Referring to FIG. 7, the reference architecture for ProSe includes E-UTRAN, EPC, plurality of UEs having ProSe applications, ProSe application server, and ProSe function. The EPC represents the E-UTRAN core network architecture. The EPC may include entities such as MME, S-GW, P-GW, policy and charging rules function (PCRF), home subscriber server (HSS), etc. The ProSe application servers are users of the ProSe capability for building the application functionality. In the public safety cases, they may be specific agencies (PSAP), or in the commercial cases social media. These applications may be defined outside the 3GPP architecture but there may be reference points towards 3GPP entities. The application server can communicate towards an application in the UE. Applications in the UE use the ProSe capability for building the application functionality. Example may be for communication between members of public safety groups or for social media application that requests to find buddies in proximity.

The ProSe Function in the network (as part of EPS) defined by 3GPP has a reference point towards the ProSe application server, towards the EPC and the UE. The functionality may include at least one of followings. But the functionality may not be restricted to the followings.

-   -   Interworking via a reference point towards the 3rd party         applications     -   Authorization and configuration of the UE for discovery and         direct communication     -   Enable the functionality of the EPC level ProSe discovery     -   ProSe related new subscriber data and handling of data storage,         and also handling of ProSe identities     -   Security related functionality     -   Provide control towards the EPC for policy related functionality     -   Provide functionality for charging (via or outside of EPC, e.g.,         offline charging)

Reference points/interfaces in the reference architecture for ProSe are described.

-   -   PC1: It is the reference point between the ProSe application in         the UE and in the ProSe application server. It is used to define         application level signaling requirements.     -   PC2: It is the reference point between the ProSe application         server and the ProSe function. It is used to define the         interaction between ProSe application server and ProSe         functionality provided by the 3GPP EPS via ProSe function. One         example may be for application data updates for a ProSe database         in the ProSe function. Another example may be data for use by         ProSe application server in interworking between 3GPP         functionality and application data, e.g., name translation.     -   PC3: It is the reference point between the UE and ProSe         function. It is used to define the interaction between UE and         ProSe function. An example may be to use for configuration for         ProSe discovery and communication.     -   PC4: It is the reference point between the EPC and ProSe         function. It is used to define the interaction between EPC and         ProSe function. Possible use cases may be when setting up a         one-to-one communication path between UEs or when validating         ProSe services (authorization) for session management or         mobility management in real time. Depending on the function         needed, PC4 may terminate in different EPC entities and may         reuse existing interfaces.     -   PC5: It is the reference point between UE to UE used for control         and user plane for discovery and communication, for relay and         one-to-one communication (between UEs directly and between UEs         over LTE-Uu).     -   PC6: This reference point may be used for functions such as         ProSe discovery between users subscribed to different PLMNs.     -   SGi: In addition to the relevant functions via SGi, it may be         used for application data and application level control         information exchange.

Applicability of the above interfaces/reference points may be dependent on solutions developed for ProSe.

For ProSe discovery, various solutions have been discussed. For one of the solutions for the ProSe discovery, network based solution for ProSe discovery has been discussed.

FIG. 8 shows ProSe architecture for network based solution for ProSe discovery. By comparing with reference architecture for ProSe described in FIG. 7 above, the description for corresponding identical components is omitted. Referring to FIG. 8, the EUTRAN is connected to the ProSe server/MME via S1-C interface. The EUTRAN is connected to the S-GW via S1-U interface. The ProSe server/MME is connected to the S-GW via S11 interface. The ProSe server/MME is connected to the HSS via S61 interface. The S-GW is connected to the P-GW via S5/S8 interface. The ProSe server functionality in the MME is utilized to authorize the ProSe discovery and/or ProSe communication. The application running in the UE asks ProSe service from the EPS layer functionality in the UE. The ProSe server allocates the ProSe ID per application instance in the UE. The ProSe ID is used for ProSe discovery and ProSe communication. The ProSe ID is sent to the application server and distributed to other users who are allowed to discover this user. New reference point U1, which is a reference point used for information exchange needed in order to perform ProSe discovery and communication between two ProSe UEs, may be used.

Two levels of authorization for ProSe services are required. At 3GPP layer, the ProSe server verifies whether a UE is subscribed to ProSe service in the 3GPP network and whether the application is allowed to use proximity service provided by the 3GPP network. Further, at the application layer, the application server authorizes whether to distribute the ProSe ID to the friends who are allowed to discover this user.

The ProSe ID is a temporary user identity specific to an application instance of which the ID is assigned by the ProSe server. Each application instance, to which proximity service can be provided, will be allocated a unique ProSe ID during the ProSe registration. The ProSe ID also indicates whether it is allocated for open discovery or restrictive discovery. For intra-MME mobility, the ProSe ID doesn't need to be reassigned. For inter-MME mobility, the application needs to perform ProSe registration and new ProSe ID is reassigned. In this case, the new ProSe ID needs to be sent to application server and distributed to the friends who are allowed to discover this user.

FIG. 9 shows a signaling flow for Prose registration procedure.

1. The UE performs LTE attach procedure after power on and establishes a PDN connection to PGW.

2. If the application running in the UE wants to use proximity service, it asks the EPS layer in the UE to perform ProSe registration procedure. The request message includes application ID and open/restrictive indication.

3. The EPS layer in the UE sends a ProSe registration request NAS signaling to the MME/ProSe server via RRC signaling. The NAS signaling includes the application ID, the globally unique temporary identifier (GUTI) and the open/restrictive indication.

4. The ProSe server verifies the subscription of the UE to see whether the indicated open/restrictive ProSe service is subscribed. The ProSe server also authorizes the application ID provided by the UE in ProSe registration request NAS signaling.

5. If the ProSe service is allowed for the UE and for the application, it asks the eNB to allocate the radio resource to the UE.

6. The ProSe server will assign a ProSe ID to the UE for this application instance. The ProSe ID indicates whether it is allocated for open/restrictive discovery. The ProSe server sends a ProSe registration acknowledge (ProSe ID) to the UE.

7. The EPS layer in the UE sends a ProSe registration complete message to the ProSe server.

8. The EPS layer in the UE sends the ProSe registration response (the ProSe ID) to the application in the UE.

9. The application in the UE registers with application server with the ProSe ID and application layer user ID.

10. The Application server stores the mapping between the ProSe ID and the application layer user ID. The ProSe ID is distributed via application layer signaling to other users which is allowed to discover this user.

FIG. 10 shows a signaling flow for targeted ProSe discovery procedure.

1. The application running in the UE1 gets the ProSe ID of the interested user via application layer signaling. If it decides to discover the proximity of the UE2, the application in the UE1 sends discovery request (ProSe ID2) to the EPS layer in the UE1.

2. The EPS layer in the UE1 makes decision whether to use open discovery or restrictive discovery based on the target ProSe ID2.

The following steps 3-9 are applied only to restrictive discovery.

3. The EPS layer in the UE1 obtains the ProSe discovery radio resource information. When the UE1 is out of coverage, it obtains this information from static configuration. When the UE1 is in coverage, it obtains this information from the network.

4. The EPS layer in the UE1 broadcasts ProSe discovery signaling (ProSe ID2, ProSe ID1). When the UE1 is in EUTRAN coverage, the discovery signaling may also be transferred via EPC.

5. The EPS layer in the UE2 receives the discovery signaling and report it to the application in the UE2 indicated by the ProSe ID2, including the source ProSe ID1.

6. The application in the UE2 checks whether it is allowed to be discovered by the UE1. If accepted, it sends discovery confirmed to the EPS layer in the UE2.

7. The EPS layer in the UE2 obtains the ProSe discovery radio resource information. When the UE2 is out of coverage, it obtains this information from static configuration. When the UE2 is in the coverage, it obtains this information from the network.

8. The EPS layer in the UE2 broadcasts ProSe discovery signaling (ProSe ID1, ProSe ID2) to confirm the ProSe discovery. When the UE2 is in EUTRAN coverage, the discovery signaling may also be transferred via EPC.

9. The EPS layer in the UE1 receives the discovery signaling and report it to the application in the UE1, indicated in step 1, including the ProSe ID2.

The following steps 10-12 are applied only to open discovery.

10. EPS layer in the UE1 obtains the ProSe discovery radio resource information.

When the UE1 is out of coverage, it obtains this information from static configuration. When the UE1 is in EUTRAN coverage, it obtains this information from the network.

11. The EPS layer in the UE1 monitors the broadcasts ProSe discovery signaling including the ProSe ID2.

12. The EPS layer in the UE1 reports to the application in the UE1, indicated in step 1, including the ProSe ID2.

When a UE is trying to discover another UE, the UE may transmit D2D discovery signals on PC5 interface, i.e., interface between ProSe UEs. In this case, when Uu interface (interface between UE and E-UTRAN) and PC5 interface are on the same frequency, interference between E-UTRAN radio and D2D (i.e., ProSe) radio may occur. Thus, the E-UTRAN may need to mitigate or avoid interference between the Uu interface and PC5 interface. But, if D2D discovery signal is transmitted by the UE in RRC_IDLE, the E-UTRAN may be unable to control interference between Uu interface and PC5 interface, or it may inefficiently coordinate the interference. On the other hand, if D2D discovery signal is transmitted by the UE in RRC_CONNECTED, the E-UTRAN may efficiently control radio transmission of D2D discovery signal and better manage the interference. However, according to the prior art, the UE may be in RRC_IDLE to perform D2D discovery (i.e., ProSe discovery).

In order to solve the problem of the prior art described above, a method for performing D2D discovery is described according to an embodiment of the present invention. According to an embodiment of the present invention, a first UE (or, first terminal/device) may discover a second UE (or, second terminal/device) based on information on the UE2 and information on D2D discovery signaling received from a network.

FIG. 11 shows an example of a method for performing D2D discovery according to an embodiment of the present invention.

In step S100, the first UE transmits a request of D2D discovery to a network. The request of D2D discovery may be transmitted if connection establishment for D2D (e.g., D2D discovery or D2D communication) is configured by the network, if the first UE is in coverage of a cell, and if there is no connection between the first UE and the network. The request of D2D discovery may be transmitted via an RRC message for establishing a connection with the cell. The RRC message may be either an RRC connection request message or RRC connection setup complete message. The RRC message may indicate that this connection establishment is for ‘D2D’, ‘D2D discovery’, or ‘D2D communication’. ‘D2D’, ‘D2D discovery’, or ‘D2D communication’ may be indicated by establishment cause in the RRC message. Further, the RRC message may include at least one of an ID of a second UE or a ProSe ID of the second UE.

Before transmitting the request of D2D discovery, the first UE may receive a request for discovering a device in proximity. The request for discovering a device may be received from an upper layer of the first UE. Alternatively, the request for discovering a device may be received from the cell via a paging message indicating D2D, e.g., D2D discovery or D2D communication. The paging message may include at least one of an ID of the first UE, a ProSe ID of the first UE, or an ID of the second UE.

In step S110, the first UE receives information on the second UE, which supports the ProSe and is located in proximity of the first UE, and information on D2D discovery signaling. The information on the second UE may include an ID of the second UE, a ProSe ID of the second UE, a sequence or a code used by the second UE, or proximity information of the second UE. The information on the D2D discovery signaling may include information on radio resources used for the D2D discovery signaling. The information on the second UE and the information on the D2D discovery signaling may be received via an RRC connection setup message or an RRC connection reject message.

In step S120, the first UE performs D2D discovery based on the received information on the second UE and information on D2D discovery signaling. That is, the first UE may transmit a signal to discover a device in proximity or to communicate with a device in proximity. More specifically, the first UE may transmit a D2D discovery signal to the second UE based on based on the received information on the second UE and information on D2D discovery signaling, and receive a D2D discovery signal from the second UE.

FIG. 12 shows another example of a method for performing D2D discovery according to an embodiment of the present invention.

In step S200, the network may configure the UE1 to initiate RRC connection establishment procedure upon receiving request for D2D discovery. In step S210, the UE1 may receive a paging message indicating D2D discovery if the UE2 is trying to discover the UE1 or another UE nearby. The D2D discovery may be indicated by a paging cause in the paging message. The paging message may include UE identity or ProSe ID of the UE1 and UE identity of the UE2.

It is assumed that the paging message indicating D2D discovery is received, or an upper layer of the UE1 requests D2D discovery. If the network configures the UE1 to initiate RRC connection establishment procedure upon receiving request for D2D discovery, if the UE1 is in coverage of a cell, and if the UE1 is in RRC_IDLE, in step S220, the UE1 determines to request D2D discovery, and in step S230, the UE1 initiates RRC connection establishment procedure by transmitting an RRC connection request message to the cell (i.e., the network). The RRC connection request message may include D2D discovery in establishment cause. However, the present invention is not limited to the RRC connection request message. If the UE1 is not in coverage of a cell, or if the UE1 is not in RRC_IDLE, the UE1 may not initiate RRC connection establishment procedure for D2D discovery. Or, if the network does not configure the UE1 to initiate RRC connection establishment procedure, the UE1 may not initiate RRC connection establishment procedure for D2D discovery.

In step S240, after transmitting the RRC connection request message to the network, the UE1 receives an RRC connection setup message or RRC connection reject message from the network. The RRC connection setup message or RRC connection reject message may include information on the UE2 (e.g., UE ID, ProSe ID, or sequence/code of the UE2), and information on D2D discovery signaling (e.g., radio resources, such as sequence/code, used for discovery signaling).

In step S250, after receiving the RRC connection setup message, the UE1 transmits an RRC connection setup complete message which may indicate D2D discovery and ProSe ID of the UE1/UE2.

In step S260, the UE1 transmits a D2D discovery signaling based on the information received from the network, e.g., via RRC connection setup message or RRC connection reject message. In step S270, upon receiving the D2D discovery signaling, the UE2 also transmits a D2D discovery signaling, e.g., based on the information received from the network.

FIG. 13 is a block diagram showing wireless communication system to implement an embodiment of the present invention.

A network 800 may include a processor 810, a memory 820 and a radio frequency (RF) unit 830. The processor 810 may be configured to implement proposed functions, procedures and/or methods described in this description. Layers of the radio interface protocol may be implemented in the processor 810. The memory 820 is operatively coupled with the processor 810 and stores a variety of information to operate the processor 810. The RF unit 830 is operatively coupled with the processor 810, and transmits and/or receives a radio signal.

A UE 900 may include a processor 910, a memory 920 and a RF unit 930. The processor 910 may be configured to implement proposed functions, procedures and/or methods described in this description. Layers of the radio interface protocol may be implemented in the processor 910. The memory 920 is operatively coupled with the processor 910 and stores a variety of information to operate the processor 910. The RF unit 930 is operatively coupled with the processor 910, and transmits and/or receives a radio signal.

The processors 810, 910 may include application-specific integrated circuit (ASIC), other chipset, logic circuit and/or data processing device. The memories 820, 920 may include read-only memory (ROM), random access memory (RAM), flash memory, memory card, storage medium and/or other storage device. The RF units 830, 930 may include baseband circuitry to process radio frequency signals. When the embodiments are implemented in software, the techniques described herein can be implemented with modules (e.g., procedures, functions, and so on) that perform the functions described herein. The modules can be stored in memories 820, 920 and executed by processors 810, 910. The memories 820, 920 can be implemented within the processors 810, 910 or external to the processors 810, 910 in which case those can be communicatively coupled to the processors 810, 910 via various means as is known in the art.

In view of the exemplary systems described herein, methodologies that may be implemented in accordance with the disclosed subject matter have been described with reference to several flow diagrams. While for purposed of simplicity, the methodologies are shown and described as a series of steps or blocks, it is to be understood and appreciated that the claimed subject matter is not limited by the order of the steps or blocks, as some steps may occur in different orders or concurrently with other steps from what is depicted and described herein. Moreover, one skilled in the art would understand that the steps illustrated in the flow diagram are not exclusive and other steps may be included or one or more of the steps in the example flow diagram may be deleted without affecting the scope and spirit of the present disclosure. 

The invention claimed is:
 1. A method for performing device-to-device (D2D) discovery in a wireless communication system, the method performed by a first user equipment (UE) which supports the D2D discovery in a radio resource control (RRC) idle state and the D2D discovery in an RRC connected state and comprising: determining whether or not the first UE is in the RRC idle state, whether or not the first UE is in coverage of a cell, and whether or not the D2D discovery and a transmission to the cell are performed on a same frequency; transmitting, to an evolved NodeB (eNB) which provides the coverage of the cell, an RRC connection request message based on the determining, wherein the RRC connection request message includes a request for the D2D discovery, and the D2D discovery is a procedure for discovering a second UE which has an ability of supporting proximity services (ProSe) and is located in proximity of the first UE; receiving information for the second UE and information for the D2D discovery in response to the RRC connection request message; and performing the D2D discovery based on the information for the second UE and the information for the D2D discovery, wherein when the first UE is determined to be in the RRC idle state, the first UE is determined to be in the coverage of the cell, and the D2D discovery and the transmission to the cell are determined to be performed on the same frequency: the first UE transmits the RRC connection request message, and the D2D discovery is performed after a transition from the RRC idle state to the RRC connected state even though the D2D discovery is supported in both the RRC idle state and the RRC connected state.
 2. The method of claim 1, wherein the request for the D2D discovery is transmitted when connection establishment for D2D is configured, and when the first UE is determined to be in the coverage of the cell.
 3. The method of claim 1, wherein the request for the D2D discovery is included in an establishment cause in the RRC connection request message.
 4. The method of claim 1, further comprising: transmitting at least one of an identity (ID) of the second UE or a ProSe ID of the second UE, to the eNB.
 5. The method of claim 1, wherein the information for the second UE includes an identity (ID) of the second UE, a ProSe ID of the second UE, a sequence or a code used by the second UE, or proximity information of the second UE.
 6. The method of claim 1, wherein the information for the D2D discovery includes information of radio resources used for the D2D discovery.
 7. The method of claim 1, wherein the information for the second UE and the information for the D2D discovery are received via an RRC connection setup message.
 8. The method of claim 1, further comprising: receiving a request for the D2D discovery from the cell via a paging message.
 9. The method of claim 8, wherein the paging message includes at least one of an identity (ID) of the first UE, a ProSe ID of the first UE, or an ID of the second UE.
 10. The method of claim 1, further comprising: receiving a request for the D2D discovery from an upper layer of the first UE.
 11. The method of claim 1, further comprising: transmitting an RRC connection setup complete message which indicates the D2D discovery to the eNB.
 12. The method of claim 11, wherein the RRC connection setup complete message includes a ProSe identity (ID) of the first UE and the second UE.
 13. The method of claim 1, wherein the performing the D2D discovery comprises: transmitting a first D2D discovery signal to the second UE based on the information for the second UE and the information for the D2D discovery; and receiving a second D2D discovery signal from the second UE.
 14. A first user equipment (UE) which supports a device-to-device (D2D) discovery in a radio resource control (RRC) idle state and the D2D discovery in an RRC connected state, the first UE comprising: a radio frequency (RF) unit that transmits or receives a radio signal; and a processor coupled to the RF unit, wherein the processor: determines whether or not the first UE is in the RRC idle state, whether or not the first UE is in coverage of a cell, and whether or not the D2D discovery and a transmission to the cell are performed on a same frequency; transmits, to an evolved NodeB (eNB) which provides the coverage of the cell, an RRC connection request message based on the determination, wherein the RRC connection request message includes a request for the D2D discovery, and the D2D discovery is a procedure for discovering a second UE which has an ability of supporting proximity services (ProSe) and is located in proximity of the first UE; receives information for the second UE and information for the D2D discovery in response to the RRC connection request message; and performs the D2D discovery based on the information for the second UE and the information for the D2D discovery, wherein when the first UE is determined to be in the RRC idle state, the first UE is determined to be in the coverage of the cell, and the D2D discovery and the transmission to the cell are determined to be performed on the same frequency: the first UE transmits the RRC connection request message, and the D2D discovery is performed after a transition from the RRC idle state to the RRC connected state even though the D2D discovery is supported in both the RRC idle state and the RRC connected state. 